The Washington Post has presumably taken the Australian search box, drawn straight lines - ideally great circles - back from either side of it towards the "last contact", and without too much difficulty plotted circles that are centered on the satellite footprint, and cross the tracks 500 miles back from the 0811 track.
Perhaps they had some official advice on whether that was appropriate or perhaps not. They do label them as provided by the NTSB, so I guess that's probably true. The Australian box extends East - so maybe that's for currents, or maybe there's something else the Australians are taking into account.
However, assuming the Australian box is right, then to be there at 0811, there isn't much option but to head there in something like a straight line.